It is usual to design a passenger tire mold cavity with a profile giving the cured tire a tri-radius crown area. Starting from center line, the first radius has the highest value so as to result in the flattest radially outer tread area in the cured tire. The second radius would permit to reach the necessary mold contour drop at the beginning of the cavity shoulder radius. This second radius has an intermediate value between the first radius and the third radius which is the shoulder radius and which permits a progressive footprint edge and a crown area junction with the tire sidewalls.
When a tire built in such a cavity is loaded on the ground, it is not difficult to flatten the first radius area. The tread portion having the second radius area which is of smaller value will resist more to be flattened to the ground. It will rather have tendency to have a hinge effect by rolling on its curvature like in a rocking chair. The “rocking chair effect” induces a lift-off of the center part and an increase in pressure in the shoulder portions as well as a slippage, which is in fact a lateral displacement in the footprint. The presence of the tire grooves in the circumferential direction will permit to avoid this rocking chair effect; however the proximity of the shoulders which are usually designed with a very low radius and have a high stiffness, will result in shifting the lift-off and the slippage to an area closer to the shoulder.
However, in the current trend to design a tread pattern for ultra high performance (UHP) tires, such circumferential grooves are normally not located in the above mentioned second radius area nor in the low meridional bending stiffness area which is in the center area. The result is an even more pronounced rocking chair effect. If the tire shoulder construction is not carefully designed (high bending stiffness), this rocking chair effect results in a poor pressure distribution. One of the effects will be a tire shoulder slippage which results in irregular wear.
U.S. Pat. No. 5,803,999 discloses a tire with a tread profile comprising a curved crown part having a radius of curvature R1 for its center portion, a pair of curbed middle portions having a radius of curvature R2 and a pair of curved shoulder portions having a radius of curvature R3 which extend continuously without forming any inflection point, wherein R1 ranges between 2 and 3.5 times the tread width TW, and is less than the radius R2 and more than the radius R3. The tread shoulder drop ranges between 0.038 and 0.050 times the tread width.
U.S. Pat. No. 5,616,195 discloses a truck tire wherein the radius R2 equals 1.3 to 2 times R1. Radius R3 equals 0.7 to 2.5 times R1. Radii R1 and R2 are convex whereas R3 is concave.
U.S. Pat. No. 5,360,044 relates to a tire with a tread surface extending along a curvature plane including a first arc with a radius of curvature R1, a second arc R2 intersecting the first arc at an intersection distance from the tire's equatorial plane by 0,2 to 0,25 times the tire width and a third arc with the radius R3 passing through a ground contact outer edge point of the ground contact surface. The curvature radius ratio R1/R2 is set in a range from 1.2 to less than 1.6. The curvature radius ratio R2/R3 is set in a range between 4 and 12.
U.S. Pat. No. 5,247,979 relates to a tire exhibiting improved handling and stability on wet roads where the net to gross is in a range comprised between 0.55 and 0.58. The radius or curvature of the shoulder of the tire R2 and the ratio of curvature of the shoulder edge portion present a ratio R2/R3 comprised between 14 and 20.
An object of the invention is to design a crown area of a tire so that the flattening on the ground can be done without a lift-off and/or a lateral displacement in the footprint. A further object of the invention is to reduce tire shoulder wear. A further object is to reduce heel and toe wear.